Hybrid Acknowledgment Request (HARQ) is a retransmission technique whereby a transmitting device, such as a base station (BS) in a wireless network, transmits redundant coded information in small increments to a receiving device, such as a mobile station (MS), subscriber station (SS), or other wireless terminal that is accessing the wireless network. In the transmitting device, a channel coder processing block encodes a data packet (P) according to a conventional algorithm, such as Viterbi encoding or turbo encoding. The output of the channel coder is the original data packet (P), plus some redundant coding data.
Next, a subpacket generator processing block generates a plurality of subpackets (SP1, SP2, SP3 . . . ) from the data packet (P) and the redundant coding data at the output of the channel coder. The transmitting device then transmits the subpackets individually and waits after each subpacket transmission for either an acknowledgment (ACK) message or a negative acknowledgment (NACK) message from the receiving device. Once the transmitting device receives from the receiving device an ACK message indicating that the original data packet (P) has been correctly received, the transmitting device stops transmitting subpackets, even if less than all of the subpackets have been sent.
By way of example, a base station that implements turbo coding generates a first set of parity bits (P1) from an original data packet (P) and generates a second set of parity bits (P2) from an interleaved version (P′) of the data packet (P). The base station then assembles the data packet (P), the first parity bits (P1), and the second parity bits (P2) into a frame of outgoing data. The outgoing frame is divided into M subpackets (SP1, SP1, SP3 . . . SPm) prior to transmission.
The subscriber station (SS) or other receiving device only needs to collect enough subpackets to correctly reassemble the original data packet (P), as indicated by a successful CRC (cyclic redundancy check). Therefore, the subscriber station transmits an ACK message only after enough subpackets have been received to correctly reassemble the original data packet (P). Prior to that, the subscriber station transmits a NACK message after each subpacket is received. By way of example, if four subpackets (SP1, SP2, SP3, SP4) are required to correctly reassemble the original data packet (P), the subscriber station transmits a NACK message after the fist three subpackets (SP1, SP2, SP3) and transmits an ACK message only after the fourth subpacket (SP4) is received.
In a multi-user environment in which a base station communicates with N subscriber stations, the base station transmits subpackets to a particular subscriber station only in selected subframes determined by a control message transmitted by the base station. The base station may transmit subpackets synchronously (e.g., every Nth subframe) or asynchronously (e.g., when ready).
In an N-channel stop-and-wait (SAW) synchronous hybrid ARQ system, N is assumed equal to 4. For synchronous HARQ, the retransmissions happen at fixed time intervals (in this example, every fourth subframe). With N=4, if the first subpacket is transmitted in subframe 1, the retransmissions can only happen in subframes 5, 9 and 13. In case of N-channel stop-and-wait (SAW), N parallel information packets can be transmitted on each of the N SAW channels.
Advantageously, in a synchronous HARQ system, the control information for the HARQ operation only needs to be transmitted along with the first subpacket transmission because the timing of the retransmissions thereafter is predetermined. However, the drawback of synchronous HARQ is that the retransmission subpackets cannot be scheduled during preferable channel conditions, because the timing of the retransmission is predetermined. Also, the modulation, coding and resource format cannot be adapted at the time of retransmission according to the prevailing channel conditions at the time of retransmission.
In an N-channel stop-and-wait (SAW) asynchronous Hybrid ARQ system, the retransmission timing, modulation, coding and resource format may be adapted according to the prevailing channel and resource conditions at the time of retransmission. One major drawback of this type of asynchronous and adaptive HARQ is that the control information needs to be sent along with all the subpackets every time. The control information transmission along with each subpacket allows adjusting the transmission timing and adjusting the new modulation, coding and resource format information. However, the transmission of control information along with each subpacket leads to excessive overhead. Also, the control information in asynchronous HARQ must be transmitted even if the transmission timing, modulation, coding and resource information are unchanged since the last subpacket transmission. This introduces redundancy in the transmission of the control information without any significant benefit.
Therefore, there is a need in the art for an improved hybrid Acknowledgment Request (HARQ) technique for use in wireless networks. In particular, there is a need for an improved HARQ technique that has the low overhead of a synchronous HARQ system and the adaptability of an asynchronous HARQ system.